Radio transmitters use filters to select a frequency range within which information modulated on a radio signal is to be transmitted. To avoid interference with transmit signals of other radio devices, mobile radio standards typically define spectral transmission masks which specify upper thresholds for the signal strength within predetermined frequency ranges. Accordingly, filters in radio transmitters may be configured to output filtered signals that comply with a given spectral transmission mask.
Mobile radio standards, for example UMTS (Universal Mobile Communications System), require mobile radio devices to transmit signals with varying strength depending on the distance of the mobile radio device to a base station. The transmit filter is supposed to have a quality such that even for signals emitted with the maximum signal strength, the spectral transmission mask is observed.
Similarly, mobile radio devices receive signals of varying strength from base stations depending on the transmission characteristics of the immediate environment and the distance of the mobile radio device to a base station. A receive filter is used in the mobile radio device to filter the receive signal within a certain frequency range and to suppress interference signals outside that frequency range (i.e. co-channel interference). The receive filter should have a quality such that even for low signal strength of the receive signal intended for the mobile radio device and/or for high co-channel interference, the filtered signal may still be decoded in the receiver.